Optical absorption spectrum in the ultra-violet and visible (UV-Vis) range of metallic nano-structures has been used as a label-free (i.e. without tagging with particles that emit light at different wavelength than the wavelength they are excited with, such as, for example fluorophores) technique for the detection of binding events in real time. A sensing mechanism is based on for example, a change in the position and/or the intensity of the spectrum or upon a change of refractive index of the medium surrounding the sensing mechanism. The refractive index of the surrounding medium has a strong influence on the absorption bands of nano-structures in the visible and near visible regions, thereby monitoring the absorption bands is highly useful for plasmonic detection of biomolecules.
A main fabrication process step of nano-biosensors is the immobilization of nano-structures on a substrate. Several techniques such as nanosphere lithography (NSL), vapor deposition, thermal evaporation and electrochemical deposition are reported for the immobilization of nanoparticles on various substrates. Using NSL, ordered nano-structures can be deposited with the help of self assembled polystyrene spheres and vacuum deposition. A widely reported method of fabrication of nano-island structures is the deposition of a thin film of gold on a substrate by vacuum deposition and annealing to yield nano-island morphology.
Presently, there is a huge demand for microfluidic devices as they are capable of performing bio-detection and drug delivery applications using small amounts (nano-liters) of bio-liquids and reagents.
Most chip-based bio-assays are based on two-dimensional (2D) chips, that is, a gold monolayer on glass having a low density of nanoparticles that are deposited on a sensing substrate and the immunoassays are carried out. Various deposition methods explained above have been reported for the fabrication of 2D chips, but these are not generally compatible with the fabrication of micro-devices. Hence new fabrication methods are essential for the development of lab-on-a-chip (LOC) devices. Integration of nano-structures in microfluidics chips is an emerging trend for the developments of high throughputs LOC devices.